A physical model to estimate snowfall over land using microwave measurements

Abstract:

Falling snow is an important component of global precipitation in extratropical regions. This study describes the methodology and results of physically based retrievals of snow falling over land surfaces. Because microwave emitted by snow-covered surfaces are highly variable, precipitating snow above such surfaces is difficult to observe using window channels that occur at low frequencies (nu < 100 GHz). Furthermore, at frequencies nu < 37 GHz, sensitivity to liquid hydrometeors is dominant. These problems are mitigated at high frequencies (nu > 100 GHz) where water vapor screens the surface emission, and sensitivity to frozen hydrometeors is significant. However, the scattering effect of snowfall in the atmosphere at those higher frequencies is also impacted by water vapor and supercooled water in the upper atmosphere.The Discrete Dipole Approximation (DDA) method was employed to generate the single scattering parameters for nonspherical snow crystals. Comparisons show that neither equivalent spheres nor dielectric mixing theories could account for all measurements. Therefore, this study builds a look up table of the DDA calculated single scattering parameters and employs it in calculations directly. Comparisons show that DDA results calculated in this study were compatible with radar and radiometer measurements for the limited number of examples.The retrieval algorithm relied on a multi-parameter cloud model to generate the vertical structure of a snow cloud, including snow water content, snow particle effective diameter, supercooled water, and water vapor. A MM5 cloud simulation was used to provide useful statistics for generating those cloud characteristics. The snow cloud profiles and surface emissivities were then used in radiative transfer calculations that were optimized against AMSU-B observations at 89, 150, and 183.3 +/- 7, +/- 3, and +/- 1 GHz. Four variables used to adjust the snow water content, relative humidity, cloud liquid water content, and surface emissivity were sufficient to estimate snowfall rates consistent with NWS radar reflectivity measurements during the New England blizzard on March 5, 2001 and to yield a Ze-M relationship that was consistent with others reported in the literature.